The field of the present invention is four-cycle engines and oil cooling systems thereof.
In the prior art an oil cooling system employing water as the cooling fluid is described in Japanese Utility Model Laid-Open No. 56-105613 (No. 104513/1981). The described system is generally constituted such that engine oil is pumped from an oil pan by an oil pump which operatively connects to the engine crankshaft and is then passed to an oil filter and fed under pressure to an oil cooler mounted integrally with the engine. Engine oil so fed to the oil cooler is thereafter supplied, after cooling to an appropriate temperature, to the several vehicle parts to be lubricated.
Also in the prior art an oil cooling system has been employed on motorcycle engines wherein the oil cooler is mounted on the forward side of the engine to the crankcase. An oil filter is also mounted on the forward side of the engine to the pan. The inlet and outlet passages communicating with the water cooled engine oil cooler are arranged for bypass of a regulated amount of oil from the inlet passage to the outlet passage to enhance the flowability of the cooled oil. Provision is made for a replaceable flow control orifice between the oil inlet and outlet passages to regulate oil viscosity. The location of the oil cooler and the oil filter at separate locations on the forward side of the engine provide for air flow past these components during movement of the vehicle to assist in the cooling effects. Reference is made to U.S. Pat. No. 4,995,448, the disclosure of which is incorporated herein by reference.
Some four-cycle engines for automobiles and motorcycles are provided with an exhaust emission control device, which supplies secondary air to the exhaust gas immediately after the same has been discharged to make the unburned gas contained in the exhaust gas burn (to be oxidized) so that the emission of the unburned gas is reduced to the lest extent.
Such an engine has previously been proposed by the applicant of the present application in Japanese Patent Laid-Open (Kokai) No. S 60-164617.
This known engine is provided with two exhaust ports provided respectively with exhaust valves for each cylinder. A secondary air feed passage is connected to one of the exhaust passages extending from the exhaust ports. Secondary air is sucked through the secondary air feed passage into the exhaust passage by the agency of a negative pressure produced by the exhaust gas flowing through the same exhaust passage. The unburned gas contained in the exhaust gas is caused to burn (to be oxidized) by the secondary air to promote controlling the exhaust emission. The exhaust passages connected respectively tot he exhaust ports are gathered at a gathering part.
When such a secondary air supply structure is employed, the unburned gas contained in the exhaust gas flowing through the other exhaust passage to which secondary air is not supplied mixes with the secondary air after passing the gathering part, and then the unburned gas is able to burn. However, since the temperature of the exhaust gas at a position after the gathering part is lower than that immediately after the same has been discharged through the exhaust port, the unburned gas is unable to burn satisfactorily and, consequently, it is highly possible that the exhaust gas having a large unburned gas content is emitted. It is also possible that the unburned gas flowing through the exhaust passage to which secondary air is not supplied does not burn satisfactorily until the temperature of the exhaust port is increased by the burning of the unburned gas in the exhaust passage to which secondary air is supplied.
A hydraulic chain tensioner, in general, applies pressure to a chain with a plunger which is biased by oil fed thereto under pressure from a lubricating circuit. In most cases, the flow rate of the oil flowing into the chain tensioner is very small. However, it is impossible to form an oil feed passage having a very small diameter because of machining difficulties. Therefore, an orifice and a leak mechanism are provided in the oil feed passage to reduce the working pressure. Such a technique is disclosed in Japanese Utility Model Laid-Open (Kokai) No. S 59-110459.
A member provided with an orifice employed in the prior art must be fabricated separately from the tensioner cylinder of the hydraulic chain tensioner and must be provided with a minute hole, and the leak groove has a complicated construction. Therefore, the member provided with the orifice is costly.